1. Field of the Invention
This invention relates to a gas sensor installed in an exhaust system of an automotive internal combustion engine for measuring a specific gas concentration in exhaust gas or the like.
2. Description of the Related Art
A gas sensor installed in an exhaust system such as an exhaust manifold or an exhaust gas pipe of an automotive combustion engine is required to have durability to harsh environment such as high temperature and hard vibration. Parts for making up the gas sensor are therefore fixed to one another by caulking and welding, because fastening with screws is insufficient to fix the parts.
The caulking fixation is processing accompanied by plastic deformation. It is known that when a processing load (caulking load) is relieved after the caulking fixation is performed, a spring-back action readily occurs to prevent a caulked fixation portion, which is fixed by caulking, from having a sufficient fastening strength. Therefore, in the caulking fixation, hot-caulking is performed after performing cold-caulking.
FIG. 8 shows a typical structure of a gas sensor 9. The gas sensor 9 is composed of a housing 2 having an insertion hole 29 into which a detection element 3 is inserted, a measurement gas side cover 111 fixed to an end portion of the housing 2, and an air side cover 121 fixed to the other end portion of the housing 2. A space defined by the detection element 3 in the insertion hole 29 is filled with powdery sealing material 132. The powdery sealing material 132 is pushed by a metallic ring 134, which is fixed to a caulked fixation portion 91 by caulking, through an insulating member 133, and the like. In the caulking fixation, specifically, the caulked fixation portion 91 is bent by caulking to hold the metallic ring 134, thereby fixing the metallic ring 134.
FIG. 9 shows the caulked fixation portion 91 before it is caulked. The caulked fixation portion 91 is integrally formed with a body portion 90 of the housing 2, and is composed of a buckling portion 911, a thick portion 913, and a caulked portion 912 provided in this order from the housing body portion 90. The caulked fixation portion 91 is caulked by cold-caulking and hot-caulking as described above, so that the detection element 3 is fixed by pressure via the metallic ring 134, the insulating member 133, and the powdery sealing material 132.
The thick portion 913 of the caulked fixation portion 91 is provided so that the deformation of the caulked portion is securely performed by caulking and so that the buckling portion is easily buckled. However, the thick portion 913 increases the weight of the housing 2, and increases the size of the gas sensor 9. The increases in size and weight are opposed to recent requirements to parts for the engine. To comply with strengthened exhaust gas regulations, every part for the engine is required to have a reduced size and a reduced weight to thereby reduce a load to the engine. The caulked fixation portion is therefore required to have a structure capable of securely performing the caulking fixation without having the thick portion. Lightening the gas sensor can additionally produce a material cost reduction.